Easy huh? The above code actually instantiates a Spirit floating point
generator (a built-in generator). Spirit has many pre-defined generators
and consistent naming conventions will help you finding your way through
the maze. Especially important to note is that things related to identical
entities (as in this case, floating point numbers) are named identically
in Spirit.Karma and in Spirit.Qi.
Actually, both libraries are using the very same variable instance to refer
to a floating point generator or parser: double_.

Now, let's create a generator that will output a line consisting of two
floating-point numbers.

double_<<double_

Here you see the familiar floating-point numeric generator double_ used twice, once for each number.
If you are used to see the '>>'
operator for concatenating two parsers in Spirit.Qi
you might wonder, what's that '<<'
operator doing in there? We decided to distinguish generating and parsing
of sequences the same way as the std::stream libraries do: we use operator
'>>' for input (parsing),
and operator '<<' for output
(generating). Other than that there is no significant difference. The above
program creates a generator from two simpler generators, glueing them together
with the sequence operator. The result is a generator that is a composition
of smaller generators. Whitespace between numbers can implicitly be inserted
depending on how the generator is invoked (see below).

Note

When we combine generators, we end up with a "bigger" generator,
but it's still a generator. Generators can get bigger and bigger, nesting
more and more, but whenever you glue two generators together, you end
up with one bigger generator. This is an important concept.

Now, creating output for two numbers is not too interesting. Let's create
a generator that will output zero or more floating-point numbers in a row.

*double_

This is like a regular-expression Kleene Star. We moved the * to the front for the same reasons we did
in Spirit.Qi: we must work with the syntax rules of
C++. But if you know regular expressions (and for sure you remember those
C++ syntax rules) it will start to look very familiar in a matter of a
very short time.

Any expression that evaluates to a generator may be used with the Kleene
Star. Keep in mind, though, that due to C++ operator precedence rules you
may need to put the expression in parentheses for complex expressions.
As above, whitespace can be inserted implicitly in between the generated
numbers, if needed.

We follow the lead of Spirit.Qi's warming up section
and will create a generator that produces a comma-delimited list of numbers.

double_<<*(lit(',')<<double_)

Notice lit(','). It is
a literal character generator that simply generates the comma ','. In this case, the Kleene Star is modifying
a more complex generator, namely, the one generated by the expression:

(lit(',')<<double_)

Note that this is a case where the parentheses are necessary. The Kleene
Star encloses the complete expression above, repeating the whole pattern
in the generated output zero or more times.

We're done with defining the generator. All that's left is to invoke the
generator to do its work. For now, we will use the generate_delimited
function. One overload of this function accepts four arguments:

An output iterator accepting the generated characters

The generator expression

Another generator called the delimiting generator

The data to format and output

While comparing this minimal example with an equivalent parser example
we notice a significant difference. It is possible (and actually, it makes
a lot of sense) to use a parser without creating any internal representation
of the parsed input (i.e. without 'producing' any data from the parsed
input). Using a parser in this mode checks the provided input against the
given parser expression allowing to verify whether the input is parsable.
For generators this mode doesn't make any sense. What is output generation
without generating any output? So we always will have to supply the data
the output should be generated from. In our example we supply a list of
double numbers as the last
parameter to the function generate_delimited
(see code below).

In this example, we wish to delimit the generated numbers by spaces. Another
generator named space is
included in Spirit's repertoire of predefined generators. It is a very
trivial generator that simply produces spaces. It is the equivalent to
writing lit(' '), or simply
' '. It has been implemented
for similarity with the corresponding predefined space
parser. We will use space
as our delimiter. The delimiter is the one responsible for inserting characters
in between generator elements such as the double_
and lit.

Ok, so now let's generate (for the complete source code of this example
please refer to num_list1.cpp).

template<typenameOutputIterator>boolgenerate_numbers(OutputIterator&sink,std::list<double>const&v){usingkarma::double_;usingkarma::generate_delimited;usingascii::space;boolr=generate_delimited(sink,// destination: output iteratordouble_<<*(','<<double_),// the generatorspace,// the delimiter-generatorv// the data to output );returnr;}

Note

You might wonder how a vector<double>, which is actually a single data
structure, can be used as an argument (we call it attribute) to a sequence
of generators. This seems to be counter intuitive and doesn't match with
your experience of using printf,
where each formatting placeholder has to be matched with a corresponding
argument. Well, we will explain this behavior in more detail later in
this tutorial. For now just consider this to be a special case, implemented
on purpose to allow more flexible output formatting of STL containers:
sequences accept a single container attribute if all elements of this
sequence accept attributes compatible with the elements held by this
container.

The generate function returns true
or false depending on the
result of the output generation. As outlined in different places of this
documentation, a generator may fail for different reasons. One of the possible
reasons is an error in the underlying output iterator (memory exhausted
or disk full, etc.). Another reason might be that the data doesn't match
the requirements of a particular generator.

Note

char and wchar_t
operands

The careful reader may notice that the generator expression has ',' instead of lit(',')
as the previous examples did. This is ok due to C++ syntax rules of conversion.
Spirit provides <<
operators that are overloaded to accept a char
or wchar_t argument on its
left or right (but not both). An operator may be overloaded if at least
one of its parameters is a user-defined type. In this case, the double_ is the 2nd argument to operator<<,
and so the proper overload of <<
is used, converting ',' into
a character literal generator.

The problem with omitting the lit
should be obvious: 'a'<<'b' is not a spirit generator, it
is a numeric expression, left-shifting the ASCII (or another encoding)
value of 'a' by the ASCII value
of 'b'. However, both lit('a')<<'b' and 'a'<<lit('b')
are Spirit sequence generators for the letter 'a'
followed by 'b'. You'll get
used to it, sooner or later.

Note that we inlined the generator directly in the call to generate_delimited. Upon calling this
function, the expression evaluates into a temporary, unnamed generator
which is passed into the generate_delimited
function, used, and then destroyed.

Here, we chose to make the generate function generic by making it a template,
parameterized by the output iterator type. By doing so, it can put the
generated data into any STL conforming output iterator.